As is known in the art, there are various means for monitoring indoor environmental or air quality parameters. One approach involves the use of facility monitoring systems that are also referred to as multipoint air monitoring systems. A multipoint air monitoring system is defined as a monitoring system that includes one or more environmental or air quality parameter sensors to measure one or more air quality parameters in a plurality of locations that includes at least one location to measure the one or more air quality parameters in at least one room, space, partially enclosed area, or environment within a building, plus at least one other location to measure the one or more air quality parameters of the supply air feeding the room, space, partially enclosed area, or environment within the building. The latter location is typically a supply air duct or outlet of the air handling unit feeding the space room, space, partially enclosed area, or environment within the building. Other types of areas may be optionally sensed such as the return air and outside air inlets of the return air handling unit feeding said room or space.
As such, a multipoint air monitoring system may involve the use of one or more individual, discrete, local, wired or wireless sensors located in the space or area being measured. It may also use remote or centralized air quality parameter sensors that are multiplexed or shared amongst a plurality of spaces. Finally, a multipoint air monitoring system may use a combination of the previously mentioned remote and local air quality parameter sensors. Many examples of multipoint air monitoring systems are disclosed in U.S. Pat. No. 8,147,302 B2 entitled “Multipoint Air Sampling System Having Common Sensors to Provide Blended Air Quality Parameter Information for Monitoring and Building Control,” which is incorporated herein by reference.
For those multipoint air monitoring systems where remote sensors are used, air is transported through a tube or pipe for sampling or measurement purposes. For example, a multipoint air monitoring system may have one or more centrally located air quality parameter sensors instead of distributed sensors local to the sensed environment. As such, this centralized air quality parameter sensor may be used in these systems to sense several or a large number of locations. These centralized air monitoring systems are also referred to as multipoint air sampling systems, or as multiplexed or shared sensor based facility monitoring systems.
Multipoint air sampling system are defined as specifically a facility monitoring system that uses shared or multiplexed sensor(s) comprising either a single remote sensor or a set of remotely located sensors used to monitor a plurality of spaces, areas or rooms within a building, or outside adjacent to a facility by transporting samples or packets of air from the spaces to be monitored to the at least one air quality parameter sensor.
For one class of these multipoint air sampling systems, a so-called star configured multipoint air sampling systems or just star configured systems, multiple tubes may be used to bring air samples from multiple locations to a centralized sensor(s). Centrally located air switches and/or solenoid valves may be used in this approach to sequentially switch the air from these locations through the different tubes to the sensor to measure the air from the multiple remote locations. Each location may be sensed for between ten seconds or several minutes. Depending on how many locations are sensed each space may be sensed on a periodic basis that could range from five to sixty minutes. These star configured systems are sometimes called octopus-like systems or home run systems and may use considerable amounts of tubing.
Systems such as this, for example, have been used to provide monitoring functions for the detection of refrigerant leaks, and other toxic gas monitoring applications. Other systems similar to this, such as that described within U.S. Pat. No. 6,241,950 to Veelenturf et al., which is incorporated herein by reference, disclose a fluid sampling system including a manifold having inputs, common purge and sampling pathways, and valves to couple/decouple first and second sets of inputs for measuring pressure differentials across sample locations.
Additionally, these types of star configured systems have been used to monitor particulates in multiple areas such as clean room areas with a single particle counter. A prior art example of this is a multiplexed particle counter such as the Universal Manifold System and Controller made by Lighthouse Worldwide Solutions, Inc., coupled with one of their particle counters such as their model number Solair 3100 portable laser based particle counter or an obscuration based particle sensor.
Regarding absolute moisture or dewpoint temperature measurement, an example of a prior art star configured multipoint air sampling system that can be used to measure dewpoint temperature is the AIRxpert 7000 Multi-sensor, Multipoint Monitoring system manufactured by AIRxpert Systems of Lexington, Mass., www.airexpert.com.
Another multipoint air sampling system defined as a networked air sampling system uses a central “backbone” tube with branches extending to various locations forming a bus-configured or tree like approach similar to the configuration of a data network. Air solenoids are typically remotely located proximate to the multiple sampling locations. The sampling time for each location like with the star configured systems may vary from about ten seconds to as much as several minutes. A typical sampling time per location would be about 45 seconds, so that with 20 locations sampled, each location could be sampled every 15 minutes. Networked air sampling systems can potentially be used to sample locations within a building, an air handling unit ductwork, exhaust air stacks of a building, or outside a building. An exemplary networked air sampling system is described in U.S. Pat. No. 6,125,710 to Sharp, which is incorporated herein by reference. U.S. Pat. No. 7,302,313 to Sharp et. al., titled “Air Quality Monitoring Systems and Methods”, references different multipoint air monitoring systems including multipoint air sampling systems as used with expert system analysis capabilities and is also incorporated herein by reference.
Another multiplexed form of facility monitoring system is defined as a networked photonic sampling system that multiplexes packets of light vs. packets of air and may incorporate either a star configured or network/bus type of layout. The basic concept uses a central laser emitter and a central laser detector that sends out and detects laser light packets that are switched into rooms to be sensed by optical switches. Optical fiber sensors, infrared absorption cells or sensors, and other sensing techniques are located and used in the sensed area to change the properties of the light due to the affect of the environment. The light packet is then switched back to the central detector where the effect of the environment on the light properties is determined. A benefit of the system is that the sensors, such as fiber or open cell sensors, are potentially quite low in cost. The expensive part is the laser and detector systems that are centralized. Similar to the previous multipoint air sampling systems, multiple affects on the light from particles, gases and other contaminants, humidity, etc. can be done simultaneously with central equipment and the telecom concept of Wavelength Division Multiplexing which allows multiple wavelengths and hence multiple signals to share the same fiber. A clear advantage of this system is the ability to have a very rapid cycle time that can be in the tens of milliseconds or less. This sampling system is detailed in U.S. Pat. No. 6,252,689, entitled “Networked Photonic Distribution System for Sensing Ambient Conditions” and is incorporated herein by reference.
The multipoint air sampling systems and networked photonic sampling system which have been described heretofore and are collectively referred to as sampling systems may be applied to monitor a wide range of locations throughout a building, including any kinds of rooms, hallways, lobbies, interstitial spaces, penthouses, outdoor locations, and any number of locations within ductwork, plenums, and air handlers. To provide control as well as monitoring of these different spaces, virtual sensor signals can be created that refer to software or firmware variables, or continuous analog or digital signals that can be passed to other systems such as a building control or laboratory airflow control system and are representative of the state of a given space's air quality parameter value. In effect these signals are reflective of what a local sensor would read if it was being used instead of the multipoint air sampling system or networked photonic sampling system otherwise known collectively again as sampling systems.
Multipoint air sampling systems have been used with a wide variety of air quality parameter sensors to monitor a wide variety of air quality attributes or air characteristics of a building or facility. An air quality parameter sensor is a sensor that can detect one or more air quality attributes or parameters that convert the level of or information about the presence of an air quality parameter into either a continuously varying or else discontinuous pneumatic, electronic, analog or digital signal or else into a software or firmware variable representing the level of or information about the presence of an air quality parameter in a given space. The air quality parameter sensor may be based on any of a variety of sensing technologies known to those skilled in the art such as for example electrochemical, photonic or optical, infrared absorption, photo-acoustic, polymer, variable conductivity, flame ionization, photo-ionization, solid state, mixed metal oxide, ion mobility, surface acoustic wave, or fiber optic. The air quality parameter sensor may be a wired or wireless sensor type and be implemented with various types of physical hardware such as for example micro-electro-mechanical system based (MEMS), nanotechnology based, micro-system based, analog based, or digital based. Additionally, an air quality parameter sensor may sense for more than one air quality parameter, and may include more than one air quality parameter sensor in a single packaged device.
An air quality parameter is defined as an air characteristic that can include an air contaminant, an air comfort parameter, or carbon dioxide (CO2). An air contaminant refers to certain potentially harmful or irritating chemical, biological, or radiological composition elements or properties of the air such as for example CO, particles of various sizes, smoke, aerosols, TVOC's (Total Volatile Organic Compounds), specific VOC's of interest, formaldehyde, NO, NOX, SOX, SO2, hydrogen sulfide, chlorine, nitrous oxide, methane, hydrocarbons, ammonia, refrigerant gases, radon, ozone, radiation, biological and or chemical terrorist agents, other toxic gases, mold, other biologicals, and other contaminants of interest to be sensed. An air contaminant specifically does not refer to such other air quality parameters such as temperature, carbon dioxide, or any one of the many forms of measuring moisture or humidity in air such as for example relative humidity, dewpoint temperature, absolute humidity, wet bulb temperature, enthalpy, etc.
Furthermore, air contaminants can be further subdivided into two categories, gas based contaminants and particle based contaminants. Gas based contaminants are defined as air contaminants that are gas or vapor based such as CO, TVOC's, ozone, etc. Particle based contaminants on the other hand include viable and nonviable air borne particulate matter of any size, but generally of a particle size from 0.01 microns up to 100 microns in diameter. As such, this category of contaminants also includes biological particulate matter such as mold spores, bacteria, viruses, etc.
If these air contaminants are generated inside a building by indoor sources then they are referred to as indoor air contaminants, such as the environmental tobacco smoke (ETS) created by indoor smokers. If the air contaminants are generated by outdoor sources, such as from road dust, automobile exhaust, or particulates generated by burning coal or other fuels, even if they are pulled into the building such as by the air handling unit they are still referred to as outdoor air contaminants.
Carbon dioxide refers specifically to the gas carbon dioxide that is found naturally in the atmosphere as a component constituent in addition to oxygen and nitrogen. It is typically found in outside air at concentrations between 300 and 500 PPM and is exhaled by human beings at an approximate rate of 0.01 CFM per person for a person doing typical office work. Variations in the number of people in an office compared to the amount of outside air supplied into the building can easily vary indoor CO2 levels to between 500 and 2500 PPM. As such CO2 can be used as an excellent indicator of proper ventilation on a per person basis sometimes referred to as the CFM of outside air per person since the level of CO2 in a space is directly related to the number of people in a space divided by the rise in CO2 from outdoor levels. Although high CO2 levels are often associated with poor indoor air quality levels, it is not the level of CO2 itself that creates the discomfort and symptoms associated with poor indoor air quality but instead the associated rise in air contaminants that are not being properly diluted. Human beings are unaffected by relatively high levels of CO2 such as up to 5000 PPM, which would be extremely rare to find in any building of ordinary construction.
An air comfort parameter specifically refers to either the measurement of temperature or one of the many related psychrometric measurements of moisture or humidity in air such as again, relative humidity, dewpoint temperature, absolute humidity, wet bulb temperature, and enthalpy. An air comfort parameter also does not refer to either carbon dioxide or any air contaminants. Additionally, an air quality parameter, air contaminant, or air comfort parameter specifically do not include any measure of airflow volume, velocity or pressure such as for example measurements of air volume that may be indicated in units of cubic feet per minute of air or other units, velocity pressure, air speed or velocity, static pressure, differential pressure, or absolute pressure.
Return air handling units are defined as air handling units that accept return air from the building where some portion of this air returned to the return air handling unit is mixed with some portion of outside air to provide a mix of return and outside air that may or may not be conditioned in some manner and then is provided as supply air to the various rooms or spaces served by the return air handling unit. The return air handling unit may or may not contain filters in the return, mixed air path, or supply airflow path that can reduce the level of air contaminants from the return air inlet of the air handler that are being delivered into the supply air stream. These filters if used may be either or both of particulate filters and gas phase filters.
The amount of air contaminants in the return air steam that pass into the supply air stream of the air handling unit will be reduced by one or both of two factors. The first factor referred to as the return air fraction is the percentage of return air that is not exhausted and is instead mixed with the outside air to constitute the supply air. For example a return air fraction of 25% would mean that 75% of the return air is exhausted and 25% is mixed with the outside air to create the return air handling unit's supply air. This means that 25% of the total contaminants in the return air stream will be fed into the supply air stream assuming no filtration.
The second factor relates to the issue of filtration. Filters located in the return, mixed air path, or supply airflow path will reduce the level of contaminants that are affected by these filters by the filtration efficiency. The filtration efficiency is defined as the percent of air contaminants that will on average be blocked by the return air handler's filters. Conversely, filtration porousness refers to the percent of air contaminants on average passing through the filters and is equal to one minus the filtration effectiveness. For example if for particulates, the return air handler has a filtration efficiency of 70% then the filtration porousness will be 30%. This means that 30% of the particulates of the return air will pass through the filter and 70% will be blocked or filtered out of the air stream. The filtration efficiency or filtration porousness can be measured for example by first measuring both the air contaminants levels entering the filter and the air contaminant levels leaving the filter. The filtration porousness is then equal to the level of contaminants leaving the filter divided by the level of contaminants entering the filter.
The term return air contaminant fraction is defined as the percentage of the total air contaminants present in the return air that will be passed into the supply air stream. For a given air contaminant, the return air contaminant fraction is equal to the product of the return air fraction and the filtration porousness for that air contaminant. When there is no filter in the return air handling unit or the filter that is used is not effective on the air contaminant (such as for a gas contaminant and a particulate filter), then the filtration porousness will be equal to one and the return air contaminant fraction will be equal to just the return air fraction.
There are many reasons that it is useful to sense the level of indoor air contaminants, such as for monitoring and safety purposes, or for the purposes of controlling the amount of dilution ventilation to eliminate or purge these contaminants from a space where they might have been generated. One known problem with sensing the level of indoor air contaminants such as for monitoring or for the control of dilution ventilation, particularly for such contaminants that are found commonly in outside air such as particles, CO, TVOC's or others, is that if the outside air concentrations become high enough, increasing the airflow volume of outside air or the supply air into a controlled area or room for purging or dilution ventilation will actually increase the sensed air contaminant levels in the controlled room or space. This can potentially create a negative feedback situation when the inside dilution ventilation threshold levels are exceeded forcing the outside airflow levels and or room supply air flow levels to their maximum level. Depending on the level of design capacity of the HVAC system, the capacity of the air handling system could be exceeded in this latch-up situation, causing a degradation of HVAC system control.
One solution to this problem of high outside air contaminant levels is disclosed in U.S. Pat. No. 8,147,302 B2 entitled “Multipoint Air Sampling System Having Common Sensors to Provide Blended Air Quality Parameter Information for Monitoring and Building Control” and is incorporated herein by reference. Rather than determining the indoor air contaminant levels by measuring the absolute level of air contaminants in a space; the '302 patent describes an approach of instead measuring the difference between the air contaminant levels in the room and the air contaminant levels present in the supply air feeding that room. In this manner, the outdoor air contaminants present in the supply air were subtracted out from the levels of air contaminants measured in the room. By this method, the amount of indoor air contaminants generated in the room was calculated.
The above approach works when the air handler that is being used is a one pass or 100% outside air unit with no return air. As such, all the supply air is outside air and the simple differential measurement of room air minus supply air is fine. This approach also works reasonably well for some, although not all, cases where a return air handling unit is involved. The problem with return air handling units is that the contaminant that may be generated in a given space will be returned to the air handling units and some percentage of this contaminant will be then mixed with the outdoor air and then fed via the return air handling unit's supply air back into the original room plus into other rooms.
If the amount of contaminant generated in a space is small compared to the total air volume of the return air handling unit, or else the period of release of the contaminant in the room is relatively short in duration such as for much less than an hour, or the numbers of rooms where this release occurs is quite small percentage-wise since the release of contaminants may be for example quite uncommon, then the amount of total contaminant in the return air and hence the supply air will also be quite small. For these cases the simple differential measurement approach of above will still work reasonably well and give indoor contaminant concentrations with good accuracy.
However, if the sources of contaminant can be large, the period of release potentially long, or the number of spaces where the contaminant is generated can be a reasonable percentage such as over 10%, then the simple differential measurement method will likely produce inaccurate results. It can also cause significant problems when used for dilution ventilation when these conditions may be present.
The reason why the results will be inaccurate, is that when the return air has a potentially significant amount of contaminants present and a reasonable percentage of these contaminants are fed into the supply air this will mean that the supply air can have reasonable levels of both outside air contaminants as well as indoor air contaminants. When the simple differential method is performed all of the supply air contaminants are subtracted from the room air contaminants, both indoor and outside air portions. Thus the total amount of indoor air contaminant in the room will not be accurately calculated since it includes both the amount instantaneously generated in the space plus the amount returned to the space. Only the amount instantaneously generated in the space will be accurately measured. Unfortunately this is not sufficient for proper dilution ventilation control. For example, assume the return air fraction is high and even if filters are being used the filter's filtration porousness for the air contaminant is also high. In this situation a low contaminant generation rate creating a low differential between the supply and room contaminant levels might not trigger the need for more outside air. However, in this situation the background levels of the contaminant could grow quite high due to insufficient outside air yet it would not be detected by the simple differential measurement method.
As shown for example by U.S. Pat. No. 8,147,302 B2, this issue of finding the true level of indoor air contaminants independent of the outdoor air contaminant in a space is also not even recognized as a problem in the prior art.
Prior art approaches do exist to determine just the outside air fraction of an air handler. Note that the return air fraction is related to the outside air fraction in that the return air fraction is equal to one minus the outside air fraction. Regarding the determination of at least the outside air fraction, it could be measured directly or a mass balance calculation could be done using temperature or another tracer compound such as carbon dioxide. For this latter case, U.S. Pat. Nos. 5,292,280 and 5,267,897 describe a multipoint air sampling system that monitors a single trace gas, typically carbon dioxide (CO2), at multiple locations, including return air, outside air, and the supply discharge air associated with an air handler in order to directly compute the outside air fraction component for the purposes of controlling the return air handling unit. This method uses a common CO2 or trace gas sensor and valves assigned to each of the sampled locations to provide a multiplexed signal from the CO2 sensor that varies in time based on the current location being sampled. The time variant signal from the shared CO2 sensor is read by a separate control module, where it is decomposed into three separate CO2 or trace gas signals, based on continuous knowledge of the sequence state, representing outside air, return air, and supply discharge air CO2 concentrations. These signals are then used in a standard mass balance equation to determine the outside air fraction.
Even if the above patents disclosed how to determine the return air fraction, it is still not enough. Additionally, the return air contaminant fraction must be calculated which may also require determining the filtration porousness of the return air handling unit filters. However even this is still not a sufficient method. This is because a fraction of the air contaminants that return to the air handler will be sent back to the room in the supply air. The room air will then include newly generated contaminants plus a fraction of the previously generated contaminants. The new combined room air will subsequently be sent back again to the return air handling unit where a portion of the combined total return air contaminants will again be fed into the supply air. This set of contaminants will again go into the room where the newly generated contaminants will be added to what is now a fraction of the previous two sets of generated contaminants. As such the return air will go around and around and the indoor air contaminant level will potentially reach some sort of an asymptotic value after some period of time. This continuing recirculation of the contaminants involving the return air contaminant fraction makes the potential solution to determine the true indoor air contaminant levels no longer a simple difference calculation.